1. Field of the Invention
This invention relates generally to blown film extrusion lines, and specifically to improved cooling systems for use with blown film systems.
2. Description of the Prior Art
Blown film extrusion lines are used to manufacture plastic bags and plastic sheets. A molten tube of plastic is extruded from an annular die, and then stretched and expanded to a larger diameter and a reduced radial thickness by the action of overhead nip rollers and internal air pressure. Typically, air is entrained by one or more blowers to provide a cooling medium which absorbs heat from the molten material and speeds up the change in state from a molten material back to a solid material. Additionally, blowers are used to provide air pressure which is utilized to control the size and thickness of the film tube. One type of blown film extrusion line utilizes an air flow on the exterior surface of the film tube in order to absorb heat. A different, and more modern, type of blown film extrusion line utilizes both an external flow of cooling air and an internal flow of cooling air in order to cool and size the film tube.
The cooling and sizing effect of these external and internal air flows is dependent upon the density of the air column and the rate of flow of the air column. These variables can be considered together in units of "mass air flow" which is simply the total density of the cooling air multiplied times the flow rate. The density of the cooling air or gas is complex and is dependent upon the relative humidity of the air or cooling gas, the absolute pressure of the air or cooling gas, the temperature of the air or cooling gas, the saturation vapor pressure of the air or cooling gas at the given temperature, the partial pressure of the water vapor in the air or cooling gas at the given temperature, and the specific gravity of the air or cooling gas. The flow rate of the air or cooling gas is of course more easily calculated.
Changes in the humidity, barometric pressure, and temperature of the ambient atmosphere will have an impact upon the cooling and gaging influence of the air or cooling gas, and will effect the product quality and production rates in a manner which is not easily calculated. The prior art systems are devoid of any useful technique or apparatus for adjusting for changes in ambient humidity, barometric pressure, and temperature in order to maintain product uniformity and to obtain the highest production rates possible.